1. Field of the Invention
The invention relates to electronic control gears for LED (light emitting diode) light engine. In particular, the electronic control gears for LED light engine use the normally closed electronic switches to gear up or down the number and current of excited LEDs in the LED array segments in accordance with the level of the AC input voltage in order to improve the power factor. Furthermore, valley fillers and/or dummy loads can be optionally added in to improve the flicker phenomenon and/or decrease the total harmonic distortion, respectively.
2. Description of the Prior Art
As compared with the traditional lighting devices, the LED has a higher luminous efficacy. The LEDs can give off more than 100 lumens per watt because less electric energy is converted into waste heat. In sharp contrast, a traditional bulb only gives off about 15 lumens per watt because more electric energy is converted into waste heat. Moreover, LED-based lighting devices are gradually becoming the preferred lighting equipment because of having a relatively longer life to reduce maintaining cost, being less susceptible to exterior interference, and being less likely to get damaged.
Technically, LEDs need to be DC-driven. So, an AC sinusoidal voltage source would normally be rectified by a full-wave or half-wave rectifier into a rectified sinusoidal voltage source before coming into use. In the vicinity of the beginning and end of each DC pulse cycle (aka “dead time”) where the input voltage is lower than the total forward voltage drop of the LEDs, the LEDs cannot be forward-biased to light up. The dead time is the partial period during which the LED current ceases conduction while the conduction angle is the partial period during which the circuit conducts the LED current. The dead time in union with the conduction angle constitutes a full period of the rectified sinusoidal voltage pulse. A longer dead time translates to a smaller conduction angle, and hence a lower power factor; more specifically, the longer the dead time, the smaller the conduction angle, and the lower the power factor, because the line current is getting too thin to be similar in shape to the line voltage. Traditional LED drivers usually come along with three application problems.
The first problem would be the need for a more complicated and more expensive driving circuit consisting of a filter, a rectifier, and a power factor corrector (PFC), etc. to drive LEDs. Besides, the short-life electrolytic capacitor used as an energy-storage component in the PFC is the key reason accounting for the shortened overall lifespan of the whole LED illumination apparatus, cancelling out the virtues of LED lighting.
The second problem would be the flicker phenomenon due to no current flowing through the LEDs during the dead time. The LEDs would immediately light up with a positive driving current, and go out with a zero driving current, causing the LEDs to flicker if there exists a dead time. If a typical AC sinusoidal frequency is 60 Hz, the rectified sinusoidal frequency will double as 120 Hz. The flicker phenomenon indeed takes place during the dead time at a repetition rate of twice the AC sinusoidal frequency although its existence might hardly be perceived by human eyes.
The third problem would be a relatively lower power factor exhibited by a low-power PFC with a loop current too weak to be precisely sensed to correctly shape the AC input current into a sinusoidal waveform. The power factor (PF) can be calculated as the input power divided by the product of the input voltage (line voltage) and the input current (line current), i.e. PF=P/(V×I), wherein P is the input power, and V and I are respectively the root-mean-square values of the line voltage and the line current. The power factor is used to measure the electricity utilization. The more similar the line current is to the line voltage, the better the electricity utilization and the higher the power factor. When the line current and the line voltage are consistent in terms of identical phase and identical shape, the power factor would reach 1 (the maximum value). The conventional PFC needs to sense its loop current for the purpose of aligning the line current with the line voltage. If the loop current goes too low to be precisely sensed by the current sense circuitry in the PFC stage, the PFC would fail to properly keep the line current in phase and in shape with the line voltage to achieve a high power factor. Often mentioned in the same breath with the issue of a low PF is the issue of a high total harmonic distortion (THD). According to the theory of Fourier series expansion of any periodic signal, any discontinuous or jumping points in the periodic waveform would incur higher-order harmonics on top of the fundamental component, causing the THD to increase. The THD resulting from the discontinuous or jumping points in the AC input current waveform would have much to do with the existence of the dead time.
Simplifying the electronic circuit, reducing the manufacturing and maintaining costs, eliminating the flicker phenomenon, as well as improving the power factor still remain the main topics put at the top of the agenda when it comes to developing new LED lighting apparatuses. The invention proposed herein to address the above issues provides an LED light engine, allowable to directly operate off of an AC power supply, in an attempt to get many benefits such as low cost, high performance, long lifespan, simple circuit topology, low flicker phenomenon, and high power factor.